A digital camera, in an existing example, captures an image using a sensor such as a charge-coupled device (CCD) or a Complementary Metal-Oxide Semiconductor (CMOS) sensor. Most of these camera sensors can read only one colour at each physical location, returning raw data as a pattern of values from each channel in a particular colour space, for instance red, green and blue (RGB), for each position or pixel in the image. For example, each R, G and B value may be represented as a number between 0 and a defined maximum such as 255, and the combination of RGB values at a pixel represents a particular colour.
The camera captures RGB values by reference to a camera colour space. However, it is rare for the camera colour space to exactly match a known reference colour space. Thus there is a difference between the “apparent colour” that a camera interprets as e.g. R=120, G=110, B=105 and the “actual colour” e.g. R=100, G=100, B=100 in a reference colour space. Therefore, colour calibration methods are needed to map apparent colour as captured by the camera to actual colour in a reference colour space. This ensures that two different devices capturing images of the same physical object report the same actual colour values. Further, lighting effects can have a very significant effect of the observed colour of an object. Most lights are not pure white, due to innate physical properties of the lights themselves, which then causes measured colours to differ from true colour.
Colour mapping is usually performed as a combination of linear and non-linear transformations of the apparent colour to actual colour by, for example, multiplication of the colour vector via an appropriate matrix and passing the individual colour values through a polynomial.
Computing such a transform is a difficult process, and requires a large number of colour samples, and preferably knowledge of the camera sensor itself. Such a technique is difficult to apply in some situations where space or time constraints render such a process impractical. Simple colour correction, such as white balance correction, is sufficient in some applications with such space or time constraints; however in some applications a higher accuracy mapping is required.
It is an aim of the present invention to provide a mapping for an image capture device that addresses one or more of the limitations of existing colour mapping methods.
The above discussion of background art is included to explain the context of the present invention. It is not to be taken as an admission that any of the documents or other material referred to was published, known or part of the common general knowledge at the priority date of any one of the claims of this specification.